1. Technical Field
The present invention relates to a liquid-crystal light-amount adjusting device that controls the amount of transmitting light, and to an apparatus for driving the liquid-crystal light-amount adjusting device. The present invention also relates to a camera in which the liquid-crystal light-amount adjusting device is used as a diaphragm.
2. Background Art
In cameras such as video cameras with built-in cameras, electronic still cameras, and still cameras, diaphragms are provided so that aperture values can be changed.
FIG. 8 is a schematic diagram showing an optical system of a conventional camera.
Referring to FIG. 8, light coming from a point 101 of a target becomes parallel beams 111 through a lens 102, and the parallel beams are partially blocked by a dual-blade diaphragm 103 and remaining beams 112 are transmitted, forming an image on an imaging surface of an imaging device 105 through a lens 104. The amount of light that transmits through the two-blade diaphragm 103 is adjusted by changing the area of a diaphragm aperture 107 by moving the blades of the two-blade diaphragm 103 by a driving apparatus 106.
The method described above, in which the amount of light that reaches the imaging device 105 from the target is adjusted by partially blocking light coming from the target by the diaphragm, causes diffraction at the edges of the blades, allowing diffracted light 113 to also reach the imaging device 105. The diffracted light 113 reaches a point that is different from the focus of the main light 112, causing degradation in resolution. The degree of degradation in resolution due to the diffraction is determined by the ratio between the amount of the light 112 that transmits through the diaphragm aperture 107 and the amount of the diffracted light 113 that diffracts at the edges of the diaphragm aperture 107, and it therefore becomes larger as the size of the camera is reduced and the size of the diaphragm is reduced accordingly. Thus, this problem is significant considering the recent miniaturization of cameras.
In order to overcome the problem, as an alternative to a blade diaphragm, a liquid-crystal light-amount adjusting device that employs liquid crystal has been proposed. The liquid-crystal light-amount adjusting device is disposed in place of a blade diaphragm. Since the optical transmittance is substantially uniform over the entire surface on which light is incident, and the optical transmittance can be controlled continuously, the liquid-crystal light-amount adjusting device allows adjustment of the amount of light without causing degradation in resolution due to diffraction. That is, the aperture value can be adjusted.
In the liquid-crystal light-amount adjusting device, in order to achieve quick response in changing the optical transmittance, a voltage that is higher (or lower) than a target voltage for achieving a specified optical transmittance (a driving voltage for acceleration) is applied to a liquid crystal for a short period. By supplying a certain electric power by the driving voltage for acceleration, a large driving force is exerted on the liquid-crystal molecules in the process of changing the orientation of the liquid crystal.
When the driving voltage for acceleration is applied by pulses, the pulses must be applied for an optimal time in order to achieve optimal response characteristics. For that purpose, the resolution increases as the frequency of the pulses becomes higher, allowing delicate control of the time of application.
However, since the liquid-crystal light-amount adjusting device is electrically a capacitor having the liquid crystal held between electrodes, a large amount of power is consumed at pulse edges if the frequency is high. In order to reduce power consumption, it is possible to reduce the current by disposing a resistor in series with the liquid crystal held between the electrodes. However, according to this method, a low-pass filter is formed by the liquid crystal and the capacitor, causing degradation in the response characteristics of the liquid crystal. Thus, from the viewpoint of reducing power consumption, the frequency of the pulses is preferably as low as possible.
As described above, the frequency of the pulses of the driving voltage for acceleration must satisfy contradicting requirements.
Accordingly, it is an object of the present invention to provide a liquid-crystal light-amount adjusting device and a liquid-crystal light-amount adjusting method that allow delicate control of a driving power for acceleration.
It is another object of the present invention to provide a camera in which the liquid-crystal light-amount adjusting device is used as a diaphragm to avoid degradation in resolution due to diffraction.